This invention relates generally to the art of ceramic heat storage devices, and, more particularly, to a magnesium oxide based cast storage device formed without the conventional step of firing.
Some European countries presently operate on an electrical power rate structure which offers reduced rates at off-peak times. Thus, during times of high electrical consumption, the cost per kilowatt hour is significantly higher than during the off-peak times. For example, peak loads occur during the time period from 5:30 a.m. to 9:00 a.m. when houses are reheated after being allowed to cool during the nighttime. Bathing and dishwashing also take place during this period of time to further increase the electrical demand.
With the advent of off-peak pricing, a financial incentive for lifestyle change came into existence. As a part of this pricing scheme, night heat storage devices came into popular use within Europe. Conventionally such heat storage devices have comprised bricks high in magnesium oxide content. Such bricks have been stacked around or assembled around an electrical heat source and permitted to accumulate heat during periods of off-peak pricing. The object of such a scheme is to have the bricks fully charged with heat at the transition from the off-peak rate to the on-peak rate. Thus fully charged night storage bricks slowly dissipate their heat throughout the on-peak time. Generally such bricks are heated in areas near the electrical resistance source to a temperature of approximately 1500.degree. to 1600.degree. F. and exhibit at the interface with the room a temperature of approximately 150.degree. F. The overall philosophy and development of night heat storage devices is described in a paper presented by H. Palmour, III et al at Cimtec 4 during 1979 at St. Vincent, Italy, and later published by Elsevier Publishing Company of Amsterdam, Holland.
As is described in the paper by Palmour, III et al these night storage bricks are formed by conventional ceramic sintering processes which require very low aluminum oxide contents in order to maintain the necessary refractoriness generally associated with ceramic products. An upper limit of 2.5 percent aluminum oxide was stated to exist for an olivine suitable for use in a night storage brick.
Olivine minerals have been used in refractory products for many years. U.S. Pat. Nos. 1,926,094; 2,252,317; 2,105,943; and 2,335,407 deal with the conversion of hydrous magnesium silicates in olivines and serpentines into the refractory mineral forsterite, 2 MgO.SiO.sub.2, which melts at about 3450.degree. F., or with the processing of these aggregates into brick or other shapes. These bricks are sintered at temperatures within the range of 2300.degree. to 2500.degree. F. for considerable periods of time, e.g., 5 hours, in order to develop sufficient mechanical strength for the article to be useful in commerce.
Harvey et al disclose in U.S. Pat. No. 2,077,795 that lime is an acceptable impurity in forsterite brick as long as the lime content does not exceed more than about 6 percent by weight. U.S. Pat. No. 2,516,249 to Osborne discloses additions of aluminum oxide to olivine in order to lessen the tendency toward bursting expansion under exposure to alternating oxidizing and reducing atmospheric conditions. This invention was commercially utilized in the steel industry until the basic slag practices became conventional. Davies et al disclose in U.S. Pat. No. 3,138,470 the use of improved forsterite aggregate. Greaves-Walker et al disclose in Bulletin No. 16, Engineering Experimental Station, North Carolina State College of Agriculture and Engineering, August 1943, the production of forsterite refractories in both the fired and unfired states utilizing various bonding agents including calcium aluminate cements. These refractories are intended for use in the steel industry and contain as a part of their constituents calcined material.